Apr 21, 2026 UT: Images of M51, ruined transit of WASP-43

Michael Richmond
Apr 21, 2026

On the night of Apr 20/21, 2026, under mostly good conditions, I acquired images of

• WASP 43 , hoping to catch an exoplanet transit (but foiled by clouds)
• spiral galaxy M51 for a class project (which turned out okay)

For the most part, the night was dark and clear. There was a 20-30 minute interruption by a cloud bank just as the ingress of WASP-43b's transit was starting, which led me to discontinue those observations. When the clouds left, I was able to acquire images of M51 through the B, V, and R filters for about 2.5 hours.

M51

These observations involved:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• B, V, R filters, with exposure time 120 seconds
• images binned 4x4
• guiding with 6-sec exposures, target at (1088, 1110) yields a bright guidestar
• focus position on 12-inch:
  • B = 0.605 yields FWHM 3.6 pixels at T = 32/0
  • V = 0.574 yields FWHM 2.5 pixels at T = 29/-2
  • R = 0.574 yields FWHM 2.5 pixels at T = 29/-2

Notes from the night:

• clear at sunset, aside from a big cloudbank low on western horizon. Clouds moved through around 10:00 - 10:30 PM, then left.
• Thin crescent Moon high in western sky at sunset, no affect on observations
• set camera to T = -15; at first, it cooled to -8.5 and then stopped, despite the cold air temperature and cooler stated to be running at 100%. When I tried to read the camera, it went into quick-readout bogus mode, so I unplugged it and re-plugged it. Afterwards, it cooled to T = -15 as usual and behaved.
• temperatures were unusually cold: T = 44/6 at sunset, dropping to 38/3 at 9 PM, 32/0 at 10 PM, and 28/-3 when I left at 1 AM
• I realize now that the camera's gain is not optimal for galaxy work. See note below.

The images I acquired do show M51, but the level of the galaxy's light appears low, not far above the sky level. This sort of faint-ish appearance of nebulosity has puzzled me for a few weeks; I recall more impressive images in years past. I think that the issue may be the camera's gain. At the moment, the FITS header reads:

  (Apr 20, 2026)

EGAIN   =  0.77999997138977051 /Electronic gain in e-/ADU                       

My suspicion is that this is set to be optimal for photometry of bright stars, when the goal is to maximize the number of electrons which can be recorded without saturating the camera. But it's not so good for faint sources: their relatively small number of photons is scaled to a small number of ADU, losing some of the dynamic range at the faint end. My guess is that modifying the gain may provide better signal-to-noise for objects with low surface brightness.

Back in 2025, when taking images of T CrB, I see that the EGAIN value had a different value:

  (Sep 11, 2025)

EGAIN   =  0.24951381981372833 /Electronic gain in e-/ADU                       

Changing the gain may also cause the flatfield exposures to yield the same, relatively high, numbers of counts that I recall from last year, for my usual exposure times. This evening, for example, a 10-second domeflat in V caused the median pixel value to be about 8000 counts; in years past, that would be closer to 20,000.

Below is one of the 120-second images in R-band.

I had to place the galaxy off-center a bit -- to the West of the optical axis -- in order to get the bright guide star in the guide camera, so one can see coma in the shapes of the stars, especially to the West (right) of the galaxy.

Note also the many single-pixel bright spots. Those should have been removed by the dark frame, but they apparently change value over time during the night.